Asymmetric modified carbon molecular sieve hollow fiber membranes having improved permeance

ABSTRACT

The present invention provides an asymmetric modified CMS hollow fiber membrane having improved gas separation performance properties and a process for preparing an asymmetric modified CMS hollow fiber membrane having improved gas separation performance properties. The process comprises treating a polymeric precursor fiber with a solution containing a modifying agent prior to pyrolysis. The concentration of the modifying agent in the solution may be selected in order to obtain an asymmetric modified CMS hollow fiber membrane having a desired combination of gas permeance and selectivity properties. The treated precursor fiber is then pyrolyzed to form an asymmetric modified CMS hollow fiber membrane having improved gas permeance.

The present application is a continuation of U.S. Non-provisionalapplication Ser. No. 14/501,884, filed on Sep. 30, 2014, which claimspriority under 35 U.S.C. § 119(e) to U.S. Provisional Application No.61/884,548, filed on Sep. 30, 2013.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to modified carbon molecularsieve (modified CMS) membranes, and more particularly to asymmetricmodified CMS hollow fiber membranes.

2. Description of the Related Art

Carbon molecular sieve (CMS) membranes have shown great potential forthe separation of gases, such as for the removal of carbon dioxide (CO₂)and other acid gases from natural gas streams. Asymmetric CMS hollowfiber membranes are preferred for large scale, high pressureapplications.

Asymmetric hollow fiber membranes have the potential to provide the highfluxes required for productive separation due to the reduction of theseparating layer to a thin integral skin on the outer surface of themembrane. The asymmetric hollow morphology, i.e. a thin integral skinsupported by a porous base layer or substructure, provides the fiberswith strength and flexibility, making them able to withstand largetransmembrane driving force pressure differences. Additionally,asymmetric hollow fiber membranes provide a high surface area to volumeratio.

Asymmetric CMS hollow fiber membranes comprise a thin and dense skinlayer supported by a porous substructure. Asymmetric polymeric hollowfibers, or precursor fibers, are conventionally formed via adry-jet/wet-quench spinning process, also known as a dry/wet phaseseparation process or a dry/wet spinning process. The precursor fibersare then pyrolyzed at a temperature above the glass transitiontemperature of the polymer to prepare asymmetric CMS hollow fibermembranes.

The polymer solution used for spinning of an asymmetric hollow fiber isreferred to as dope. During spinning, the dope surrounds an interiorfluid, which is known as the bore fluid. The dope and bore fluid arecoextruded through a spinneret into an air gap during the “dry-jet”step. The spun fiber is then immersed into an aqueous quench bath in the“wet-quench” step, which causes a wet phase separation process to occur.After the phase separation occurs, the fibers are collected by a take-updrum and subjected to a solvent exchange process.

The solvent exchange process is an extremely important step in themembrane fabrication process. If the porous precursor fibers containwater at the time they are subjected to high temperatures, for instanceduring drying or pyrolysis, removal of the water causes significantchanges to the structure and properties of the fiber and of theresulting CMS membrane. The high capillary forces associated withremoval of water within the small radii of the pores close to the skincause a densification of the structure in this region, which results ina less permeable membrane. To prevent this, the solvent exchange processreplaces the water that is present in the porous substructure of theprecursor fiber with a fluid having a lower surface tension.

A conventional solvent exchange process involves two or more steps, witheach step using a different solvent. The first step or series of stepsinvolves contacting the precursor fiber with one or more solvents thatare effective to remove the water in the membrane. This generallyinvolves the use of one or more water-miscible alcohols that aresufficiently inert to the polymer. The aliphatic alcohols with 1-3carbon atoms, i.e. methanol, ethanol, propanol, isopropanol, andcombinations of the above, are particularly effective as a firstsolvent. The second step or series of steps involves contacting thefiber with one or more solvents that are effective to replace the firstsolvent with one or more volatile organic compounds having a low surfacetension. Among the organic compounds that are useful as a second solventare the C₅ or greater linear or branched-chain aliphatic alkanes.

The solvent exchange process typically involves soaking the precursorfibers in a first solvent for a first effective time, which can range upto a number of days, and then soaking the precursor fibers in a secondsolvent for a second effective time, which can also range up to a numberof days. Where the precursor fibers are produced continuously, such asin a commercial capacity, a long precursor fiber may be continuouslypulled through a series of contacting vessels, where it is contactedwith each of the solvents. The solvent exchange process is generallyperformed at room temperature.

The precursor fibers are then dried by heating to temperature above theboiling point of the final solvent used in the solvent exchange processand subjected to pyrolysis in order to form asymmetric CMS hollow fibermembranes.

The choice of polymer precursor, the formation and treatment of theprecursor fiber prior to pyrolysis, and the conditions of the pyrolysisall influence the performance properties of an asymmetric CMS hollowfiber membrane.

Important properties of asymmetric CMS hollow fiber membranes includepermeance and selectivity. Permeance measures the pressure-normalizedflux of a given compound while selectivity measures the ability of onegas to permeate through the membrane versus a different gas. Theseproperties, and the methods by which asymmetric CMS hollow fibermembranes may be tested to determine these properties, are described inmore detail in, for example, U.S. Pat. Nos. 6,565,631 and 8,486,179, thecontents of both of which are hereby incorporated by reference.

Though asymmetric CMS hollow fiber membranes exhibit encouragingselectivities, they exhibit lower permeance after pyrolysis than wouldbe expected based on the permeability increase in corresponding densefilms before and after pyrolysis of the same precursor polymer. Thelower than expected permeance is thought to be caused, at least in part,by a phenomenon known as substructure morphology collapse.

As described in U.S. patent application Ser. No. 13/666,370, thecontents of which are hereby incorporated by reference, substructuremorphology collapse occurs when intensive heat-treatment duringpyrolysis relaxes the polymer chains, causing their segments to movecloser to one another and collapsing the pores in the substructure. Thissubstructure morphology collapse results in an increased actual membraneseparation thickness, and thus a drop in permeance. Because of thepermeance drop, the advantage of having a high transport flux in anasymmetric fiber is lost significantly.

In U.S. patent application Ser. No. 13/666,370, Bhuwania et al.described a method for treating precursor fibers in order to limit thesubstructure collapse that occurs during pyrolysis. Bhuwania et al.showed that by soaking the precursor fibers in a chemical modifyingagent, such as vinyl trimethoxy silane (VTMS), before pyrolysis,asymmetric CMS hollow fibers having an increased permeance could beformed. Without being bound by any theory, Bhuwania et al. describedthat the chemical modifying agent thermally and/or physically stabilizesthe precursor fiber prior to pyrolysis.

It has now surprisingly been found that by contacting a precursor fiberwith a solution containing the modifying agent at a concentration ofless than 100%, the permeance of the resulting asymmetric modified CMShollow fiber membrane can be increased to a degree well beyond thatwhich is achieved by soaking the precursor fiber in the chemicalmodifying agent alone, as was described in U.S. patent application Ser.No. 13/666,370, without having an adverse effect on the selectivity ofthe modified CMS hollow fiber membrane.

SUMMARY OF THE INVENTION

It is an object of at least one embodiment of the present invention toprovide a process for preparing an asymmetric modified CMS hollow fibermembrane having improved gas separation performance properties bytreating a polymeric precursor fiber with a solution containing amodifying agent prior to pyrolysis. The concentration of the modifyingagent in the solution may be selected in order to obtain an asymmetricmodified CMS hollow fiber membrane having a desired combination of gaspermeance and selectivity properties. The precursor fiber is thenpreferably contacted with a moisture-containing atmosphere. The treatedprecursor fiber is pyrolyzed to form an asymmetric modified CMS hollowfiber membrane having improved gas permeance.

For example, the concentration of the modifying agent in the solutionmay be selected to obtain an asymmetric modified CMS hollow fibermembrane having a gas permeance property that is at least a 400%increase over an equivalent asymmetric CMS hollow fiber membrane thatwas not subjected to treatment with the modifying agent. Theconcentration of the modifying agent in the solution may also beselected to obtain an asymmetric modified CMS hollow fiber membrane thatis useful for the separation of particular components within a gasstream. For example, the concentration of the modifying agent may beselected to obtain an asymmetric modified CMS hollow fiber membrane thatis useful for the separation of acid gases, such as CO₂ and H₂S, from ahydrocarbon-containing gas stream such as natural gas. The concentrationof the modifying agent may also be selected to obtain an asymmetricmodified CMS hollow fiber membrane that is configured for the separationof particular gases, including but not limited to CO₂ and CH₄, H₂S andCH₄, CO₂/H₂S and CH₄, CO₂ and N₂, O₂ and N₂, N₂ and CH₄, He and CH₄, H₂and CH₄, H₂ and C₂H₄, ethylene and ethane, propylene and propane, andethylene/propylene and ethane/propane, each of which may be performedwithin a gas stream comprising additional components.

It is also an object of at least one embodiment of the present inventionto provide a process for preparing an asymmetric modified CMS hollowfiber membrane having an improved gas permeance property by treating aprecursor fiber with a solution containing a modifying agent, in whichthe modifying agent is present in the solution at a concentrationbetween about 1 and about 90 percent by weight, and then pyrolyzing thetreated fibers to form an asymmetric modified CMS hollow fiber membrane.

In another aspect, it is an object of at least one embodiment of thepresent invention to provide a process for forming an asymmetricmodified CMS hollow fiber membrane in which at least one of the solventexchange materials with which the precursor fiber is contacted prior topyrolysis contains a modifying agent in an amount that is effective toimprove the gas permeance of the asymmetric modified CMS hollow fibermembrane. In the preparation of an asymmetric CMS hollow fiber membrane,the polymeric hollow fiber is spun and then immersed in an aqueousbath—a process known as the dry-jet, wet-quench method. Then, in asolvent exchange step, the fiber is contacted with an organic compoundhaving a low surface tension, such as n-hexane, which enters the poresof the fiber. By replacing the organic compound of the conventionalsolvent exchange process with a solution comprising the organic compoundand a modifying agent, it has now been found that pyrolysis of thetreated precursor fibers produces asymmetric modified CMS hollow fibermembranes having improved gas permeance properties.

Accordingly, at least one embodiment of the present invention isdirected to a process for forming an asymmetric modified CMS hollowfiber membrane that includes forming an asymmetric hollow polymer fiber,contacting the hollow polymer fiber with a solvent exchange materialcomprising a modifying agent in an amount effective to improve the gaspermeance of the asymmetric CMS hollow fiber membrane, and pyrolyzingthe hollow polymer fiber to form an asymmetric modified CMS hollow fibermembrane. At least another embodiment of the present invention isdirected to a process for preparing an asymmetric polymer precursorfiber by the dry-jet, wet-quench method, the improvement comprisingduring the solvent exchange step, contacting the fiber with a modifyingagent that is effective to increase the gas permeance of the asymmetricmodified CMS hollow fiber membrane that is formed upon pyrolysis of theasymmetric polymer precursor fiber.

It is another object of at least one embodiment of the present inventionto provide an asymmetric modified CMS hollow fiber membrane having amorphology stabilizer within at least one of its pores. Treatment of apolymer precursor fiber with a modifying agent prior to pyrolysis causesa sol-gel reaction to take place between the modifying agent and themoisture contained within the pores of the polymeric precursor fiber.During pyrolysis, the sol-gel is converted to a morphology stabilizer,which acts to support the pores and restrict collapse. In at least onepreferred embodiment, the morphology stabilizer comprises asilicon-containing compound, such as one containing siloxane bridges. Inat least one embodiment, the amount of morphology stabilizer within theasymmetric modified CMS hollow fiber membrane can be generally measuredby elemental analysis. Preferably, the asymmetric modified CMS hollowfiber membrane contains between about 0.1 and about 10 mol % of anindicating element, such as silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

A clear conception of the advantages and features of one or moreembodiments will become more readily apparent by reference to theexemplary, and therefore non-limiting, embodiments illustrated in thedrawings:

FIG. 1 is an illustration of an exemplary precursor fiber treatmentprocess according to various embodiments of the present invention.

FIG. 2 is an illustration of a reaction of the type that is believed totake place when a precursor fiber is contacted with a modifying agentaccording to various embodiments of the present invention.

FIG. 3 is an illustration of a film of the type that is believed to formon the outer skin layer of an asymmetric hollow precursor fiber when theprecursor fiber is contacted with a modifying agent at highconcentrations.

FIG. 4 shows test results of a dynamic mechanical analysis,demonstrating the loss of storage modulus observed on heating ofMatrimid® 5218 precursor fibers to their glass transition temperature(T_(g)) and the restriction of such loss observed with Matrimid® 5218precursor fibers treated according to various embodiments of the presentinvention.

FIG. 5A shows test results of ²⁹Si solid state nuclear magneticresonance (NMR) demonstrating the presence of siloxane bridges in aprecursor fiber treated according to embodiments of the presentinvention.

FIG. 5B shows test results of ²⁹Si solid state nuclear magneticresonance (NMR) demonstrating the presence of siloxane bridges in anasymmetric carbon molecular sieve hollow fiber treated according toembodiments of the present invention.

FIG. 6 shows test results of ¹³C solution nuclear magnetic resonance(NMR) demonstrating that the modifying agent does not react with polymerprecursor fiber.

FIG. 7 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid® 5218 precursor fiber with a solutioncomprising 75 percent by weight (75 wt %) VTMS and pyrolyzed at 550° C.

FIG. 8 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid® 5218 precursor fiber with a solutioncomprising 50 percent by weight (50 wt %) VTMS and pyrolyzed at 550° C.

FIG. 9 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid precursor fiber with a solutioncomprising 25 percent by weight (25 wt %) VTMS and pyrolyzed at 550° C.

FIG. 10 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid® 5218 precursor fiber with a solutioncomprising 10 percent by weight (10 wt %) VTMS and pyrolyzed at 550° C.

FIG. 11 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid® 5218 precursor fiber with a solutioncomprising 5 percent by weight (5 wt %) VTMS and pyrolyzed at 550° C.

FIG. 12 shows SEM images of asymmetric modified CMS hollow fibersprepared by contacting a Matrimid® 5218 precursor fiber with a solutioncomprising 1 percent by weight (1 wt %) VTMS and pyrolyzed at 550° C.

FIG. 13 is a graphical representation of test results demonstratingexemplary improved gas separation properties for asymmetric modified CMShollow fibers prepared according to embodiments of the presentinvention.

FIG. 14 is an illustration of an exemplary pyrolysis process that can beused with various embodiments of the present invention.

FIG. 15A shows the CO₂ permeance of exemplary asymmetric modified CMShollow fiber membranes (prepared by treating Matrimid 2158® precursorfibers with a solution containing 10 wt % VTMS and pyrolyzing at twodifferent temperatures) in a mixed gas containing 50 mole % CO₂ and 50mole % CH₄ and at pressures up to 800 pounds per square inch (psi).

FIG. 15B shows the CO₂/CH₄ selectivity of exemplary asymmetric modifiedCMS hollow fiber membranes (prepared by treating Matrimid 2158®precursor fibers with a solution containing 10 wt % VTMS and pyrolyzingat two different temperatures) in a mixed gas containing 50 mole % CO₂and 50 mole % CH₄ and at pressures up to 800 pounds per square inch(psi).

DETAILED DESCRIPTION OF THE INVENTION

Asymmetric Modified CMS Hollow Fiber Membranes and MorphologyStabilizers

An asymmetric modified CMS hollow fiber membrane is an asymmetric CMShollow fiber membrane that has been treated with a modifying agent priorto pyrolysis such that substructure collapse of the fiber duringpyrolysis is limited, bringing about an increase in the gas permeance ofthe asymmetric CMS hollow fiber membrane over one that has been preparedin the same manner but without being treated with the modifying agent.

Treatment of the precursor fiber with a modifying agent also alters theelemental makeup of the asymmetric CMS hollow fiber membrane. Forexample, the modifying agent may contain elements, such as silicon,metals or combinations thereof, whose presence decreases the weightpercent of carbon in a modified CMS hollow fiber membrane. A modifiedCMS hollow fiber membrane may comprise, for instance, between about 60%and about 80% by weight carbon, compared with a conventional CMS hollowfiber membrane, which typically comprises at least 80% by weight carbon.A modified CMS hollow fiber membrane is not defined by the amount orpercentage of carbon in its elemental makeup and does not require aparticular minimum amount or percentage of carbon to be present.

It has now been found that the modifying agent need not react with thepolymer precursor fiber itself, but rather that the modifying agent mayreact with moisture that is present in the pores of a precursor fiber ormolecularly sorbed between the polymer chain segments. It is believedthat the modifying agent reacts with the moisture that is present in thepores of the precursor fiber by a sol-gel reaction process to form asolid morphology stabilizer structure. FIG. 1 illustrates the two stepsof the contemplated sol-gel reaction, in which vinyltrimethoxysilane(VTMS), a preferred modifying agent, is converted into a morphologystabilizer within the pores of an asymmetric hollow fiber. In a firststep, the modifying agent undergoes hydrolysis and polycondensationreactions to form a chain-like network. The first step is illustrated,for example, in FIG. 2, which shows the contemplated hydrolysis andcondensation reactions of vinyltrimethoxysilane (VTMS) to form achain-like network. It is believed that the first step, i.e. thereaction of the modifying agent to form a chain-like network, is broughtabout by contacting a precursor fiber with the modifying agent, such asby soaking the precursor fiber in a solution containing the modifyingagent, and then contacting the precursor fiber with moisture, such as byplacing the fiber under a moist atmosphere. In a second step, thechain-like network is converted to a solid structure. As illustrated inFIG. 1, it is believed that the second step occurs during pyrolysis ofthe precursor fiber.

The result of the reaction is the formation of a morphology stabilizer.The term morphology stabilizer as used herein refers to the residue of amodifying agent that acts to restrict pore collapse during pyrolysis andthat resides in the porous substructure of the asymmetric modified CMShollow fiber. The morphology stabilizer is a glassy structure that actsas a sort of scaffolding within a pore, thereby preventing the collapseof the pore. In some embodiments, the morphology stabilizer may beporous. Where the morphology stabilizer is porous, such as a morphologystabilizer comprising mesopores, its interference with the flow of gasthrough the modified CMS hollow fiber membrane is reduced.

The formation of a morphology stabilizer through a sol-gel reactionprocess, as well as the relationship between the morphology stabilizerand the modified CMS hollow fiber membrane, has been discovered andconfirmed through a number of studies.

Example 1

Matrimid® 5218 precursor fibers were soaked in pure VTMS at roomtemperature for about twelve hours. The fibers were then removed andplaced into a glove bag saturated with air of relative humidity at about100%. After about forty-eight hours, the fibers were removed from theglove bag and dried under vacuum at 150° C. for about twelve hours. Aportion of the treated Matrimid® 5218 precursor fibers were reserved fortesting. The remainder of the treated Matrimid® 5218 precursor fiberswere pyrolyzed under an atmosphere of ultra-high purity argon (˜99.9%)at a maximum pyrolysis temperature of about 650° C. The fibers were heldat the maximum pyrolysis temperature for about two hours.

Both the treated Matrimid® 5218 precursor fibers, i.e. the pre-pyrolysisfibers, and the modified CMS hollow fibers, i.e. the post-pyrolysisfibers, were tested by ²⁹Si solid state nuclear magnetic resonance(NMR). After treatment with VTMS, the Matrimid® 5218 precursor fibersexhibited peaks that were indicative of siloxane bonds, also sometimesreferred to as siloxane bridges. These siloxane bridges are indicativeof the hydrolysis and condensation of the VTMS via a sol-gel reaction.After being subjected to pyrolysis, the CMS hollow fibers also exhibitedpeaks that were indicative of siloxane bridges. ²⁹Si solid state NMRdemonstrates that the asymmetric modified CMS hollow fibers comprise aresidue of the sol-gel reaction. The results of this study are shown inFIGS. 5A and 5B.

Example 2

Matrimid® 5218 precursor fibers were soaked in pure VTMS at roomtemperature for about twelve hours. The fibers were then removed andplaced into a glove bag saturated with air of relative humidity at about100%. After about forty-eight hours, the fibers were removed from theglove bag and dried under vacuum at 150° C. for about twelve hours.

The treated Matrimid® 5218 precursor fibers were tested by ¹³C solutionnuclear magnetic resonance (NMR) and the results compared against the¹³C solution nuclear magnetic resonance spectrum for untreated Matrimid®5218 precursor fibers. The results are shown in FIG. 6. Notably, the ¹³Csolution NMR spectrum of the treated Matrimid® 5218 precursor fibers hasno substantial difference from the ¹³C solution NMR spectrum of theMatrimid® 5218 precursor fibers that were not contacted with the VTMSmodifying agent. These results indicate that the modifying agent doesnot react with the Matrimid® 5218 precursor fiber, i.e. that the sol-gelreaction did not occur between the modifying agent and the polymerprecursor. Rather, it was determined that the sol-gel reaction occurredbetween the modifying agent and moisture that is present within thepores of the Matrimid® 5218 precursor fibers.

Precursor Fibers

The asymmetric polymer precursor fiber may comprise any polymericmaterial that, after undergoing pyrolysis, produces a CMS membrane thatpermits passage of the desired gases to be separated, for example carbondioxide and natural gas, and in which at least one of the desired gasespermeates through the CMS fiber at different diffusion rate than othercomponents. The polyimides are preferred polymers precursor materials.Suitable polyimides include, for example, Ultem® 1000, Matrimid® 5218,6FDA/BPDA-DAM, 6FDA-6FpDA, and 6FDA-IPDA.

The polyimide commercially sold as Matrimid® 5218 is a thermoplasticpolyimide based on a specialty diamine,5(6)-amino-I-(4′aminophenyl)-I,3,-trimethylindane. Its structure is:

The Matrimid® 5218 polymers used in the Examples were obtained fromHuntsman International LLC. 6FDA/BPDA-DAM is a polymer made up of2,4,6-Trimethyl-1,3-phenylene diamine (DAM), 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA), and5,5-[2,2,2-trifluoro-I-(trifluoromethyl)ethylidene]bis-I,3-isobenzofurandione(6FDA), and having the structure:

To obtain the above mentioned polymers one can use available sources orsynthesize them. For example, such a polymer is described in U.S. Pat.No. 5,234,471, the contents of which are hereby incorporated byreference.

Examples of other suitable precursor polymers include polysulfones;poly(styrenes), including styrene-containing copolymers such asacrylonitrilestyrene copolymers, styrene-butadiene copolymers andstyrene-vinylbenzylhalide copolymers: polycarbonates; cellulosicpolymers, such as cellulose acetate-butyrate, cellulose propionate,ethyl cellulose, methyl cellulose, nitrocellulose, etc.; poly-amides andpolyimides, including aryl polyamides and aryl polyimides; polyethers;polyetherimides; polyetherketones; poly(arylene oxides) such aspoly(phenylene oxide) and poly(xylene oxide);poly(esteramide-diisocyanate); polyurethanes; polyesters (includingpolyarylates), such as poly(ethylene terephthalate), poly(alkylmethacrylates), poly(acrylates), poly(phenylene terephthalate), etc.;polypyrrolones; polysulfides; polymers from monomers havingalpha-olefinic unsaturation other than mentioned above such aspoly(ethylene), poly(propylene), poly(butene-I), poly(4-methylpentene-1), polyvinyls, e.g., poly(vinyl chloride), poly(vinylfluoride), poly(vinylidene chloride), poly(vinylidene fluoride),poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl acetate) andpoly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones),poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes) such aspoly(vinyl formal) and poly(vinyl butyral), poly(vinyl amides),poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinylphosphates), and poly(vinyl sulfates); polyallyls;poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles;polytriazoles; poly(benzimidazole); polycarbodiimides; polyphosphazines;etc., and interpolymers, including block interpolymers containingrepeating units from the above such as terpolymers ofacrylonitrile-vinyl bromide-sodium salt of para-sulfophenylmethallylethers; and grafts and blends containing any of the foregoing. Typicalsubstituents providing substituted polymers include halogens such asfluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups;lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.

Preferably, the polymer is a rigid, glassy polymer at room temperatureas opposed to a rubbery polymer or a flexible glassy polymer. Glassypolymers are differentiated from rubbery polymers by the rate ofsegmental movement of polymer chains. Polymers in the glassy state donot have the rapid molecular motions that permit rubbery polymers theirliquid-like nature and their ability to adjust segmental configurationsrapidly over large distances (>0.5 nm). Glassy polymers exist in anon-equilibrium state with entangled molecular chains with immobilemolecular backbones in frozen conformations. The glass transitiontemperature (Tg) is the dividing point between the rubbery or glassystate. Above the Tg, the polymer exists in the rubbery state; below theTg, the polymer exists in the glassy state. Generally, glassy polymersprovide a selective environment for gas diffusion and are favored forgas separation applications. Rigid, glassy polymers describe polymerswith rigid polymer chain backbones that have limited intramolecularrotational mobility and are often characterized by having high glasstransition temperatures. Preferred polymer precursors have a glasstransition temperature of at least 200° C.

In rigid, glassy polymers, the diffusion coefficient tends to controlselectivity, and glassy membranes tend to be selective in favor ofsmall, low-boiling molecules. For example, preferred membranes may bemade from rigid, glassy polymer materials that will pass carbon dioxide,hydrogen sulfide and nitrogen preferentially over methane and otherlight hydrocarbons. Such polymers are well known in the art and includepolyimides, polysulfones and cellulosic polymers.

The asymmetric polymer precursor fiber may be a composite structurecomprising a first polymer material supported on a porous second polymermaterial. Composite structures may be formed by using more than onepolymer material as the dope during the asymmetric hollow fiber spinningprocess.

In some embodiments, the polymer precursor fiber may contain functionalreactive groups that react with the modifying agent. As demonstrated inExample 2, reaction of the modifying agent and the polymer precursorfiber is not necessary for the formation of either a morphologystabilizer or a modified CMS hollow fiber membrane. However, it iscontemplated that some precursor polymer materials may react with themodifying agent. For example, precursors prepared using polymermaterials that contain hydroxyl (—OH) groups or acid (such as —COOH)functional groups may react with the modifying agent. It is contemplatedthat this reaction may take place in addition to the sol-gel reactionbetween the modifying agent and moisture within the pores, and thatpyrolysis will still result in a morphology stabilizer and an asymmetricmodified CMS hollow fiber membrane.

Modifying Agents

The term modifying agent, as used herein, refers to a compound that iscapable of undergoing a reaction within the pores of a polymer precursorfiber to form a morphology stabilizer without otherwise adverselyaffecting the mechanical properties of the fiber.

Preferred modifying agents are those that undergo a polycondensationreaction to form siloxane bridges. For example, the modifying agent maybe a silane having the general formula R¹R²R³R⁴Si, where each of R¹, R²,R³, and R⁴ are independently C₁-C₆ alkyl or alkenyl, alkoxy, or halogen,with the proviso that the silane contains at least one C₁-C₆ alkyl oralkenyl substituent and at least one alkoxy or halogen substituent. Theat least one alkoxy or halogen substituent provides the silane with thecapability of forming a chain-like network of siloxane bonds. The atleast one C₁-C₆ alkyl or alkenyl substituent provides that the treatmentof a fiber with the modifying agent does not render the fiber brittle.Subject to this proviso, each of the substituents can be varied in orderto provide the silane with desired properties. For example, by selectionof the substituent groups, one may be able to alter the porosity of theresulting morphology stabilizer.

In some preferred embodiments, vinyl trimethoxy silane (VTMS) is used asthe modifying agent for precursor treatment, but other silanes can alsobe employed as a modifying agent. The modifying agent, for example, maybe a monosilane or an oligosiloxane such as a disiloxane or atrisiloxane. For instance, in various embodiments, the modifying agentmay be selected from the group consisting of vinyl trimethoxysilane,vinyl triethoxysilane, vinyl dimethoxychlorosilane, vinyldiethoxychlorosilane, vinyl methoxydichlorosilane, vinyl ethoxydichlorosilane, vinyl trichloro silane, vinyl pentamethoxydisiloxane, divinyltetramethoxydisiloxane, and combinations thereof. In variousparticularly preferred embodiments, the at least one alkoxy or halogensubstituent comprises methoxy or ethoxy. In various particularlypreferred embodiments, the at least one C₁-C₆ alkyl or alkenylsubstituent comprises vinyl. Particularly preferred modifying agentsinclude vinyl trimethoxy silane, vinyl triethoxy silane, ethanetrimethoxy silane, and methyl trimethoxy silane.

Other modifying agents include those that undergo a polycondensationreaction to form metal-oxo and/or metal oxycarbide bonds. For example,the modifying agent may be a metal alkoxide having the general formulaR¹R²R³R⁴M, where M is a metal and where each of R¹, R², R³, and R⁴ areindependently C₁-C₆ alkyl or alkenyl, alkoxy, or halogen, with theproviso that the metal alkoxide contain at least one C₁-C₆ alkyl oralkenyl substituent and at least one alkoxy or halogen substituent. Theat least one alkoxy or halogen substituent provides the metal alkoxidewith the capability of forming a chain-like network of metal-oxo and/ormetal oxycarbide bonds. The at least one C₁-C₆ alkyl or alkenylsubstituent provides that the treatment of a fiber with the metalalkoxide does not render the fiber brittle. Subject to this proviso,each of the substituents can be varied in order to provide the metalalkoxide with desired properties. For example, by selection of thesubstituent groups, one may be able to alter the porosity of theresulting morphology stabilizer. In preferred embodiments, the metal Mis selected from the group consisting of Ge, B, Al, Ti, V, Fe, andcombinations thereof.

Treatment and Pyrolysis Conditions

In modifying a polymer precursor fiber to prepare a substantiallynon-collapsed, asymmetric modified CMS hollow fiber membrane, theprocess comprises the steps of providing the polymer precursor,providing a contacting solution comprising a modifying agent (which ispresent in the solution at a concentration of less than 100 wt %), andallowing at least a portion of the polymer precursor to contact at leasta portion of the contacting solution comprising the modifying agent tocreate a modified polymer precursor that, when pyrolyzed, produces asubstantially non-collapsed, asymmetric modified CMS hollow fibermembrane. Preferably, the polymer precursor is soaked in a solutioncomprising the modifying agent at a desired concentration for a periodof time sufficient to allow the modifying agent to enter thesubstructure pores of the precursor fiber. Preferably, the period oftime is from about 30 minutes to about 24 hours.

The solution containing the modifying agent need not contact an end ofthe hollow precursor fiber in order to enter the substructure pores ofthe precursor fiber. Rather, it has been found that the modifying agentmay penetrate the outer skin of the precursor fiber in a radialdirection, and enter the substructure pores of the fiber in this manner.

The contacting of a precursor fiber with a modifying agent preferablytakes place at room temperature. However, in some additionalembodiments, the contacting temperature may be held within a rangeselected from approximately 20° C. to approximately the polymerprecursor glass transition temperature; from approximately 100° C. toapproximately the polymer precursor glass transition temperature; andfrom approximately 100° C. to approximately 250° C.

In various embodiments, the reaction of the modifying agent to form amorphology stabilizer may require the addition of a catalyst. Forexample, when vinyl triethoxy silane is used as the modifying agent, itmay be desirable to add a catalyst to promote the sol-gel reaction. Thisis due to the slow reaction of ethoxy groups compared to the methoxygroups of, for example, VTMS. The sol-gel reaction can be promotedthrough the addition of an acid, such as a mineral acid, as it is knownin the art that a sol-gel reaction is often significantly increasedunder acidic conditions. Preferred acid catalysts include any readilyavailable mineral acid, such as hydrochloric acid, sulfuric acid, nitricacid, phosphoric acid, boric acid, hydrofluoric acid, hydrobromic acid,perchloric acid, and combinations thereof.

Once the precursor fiber has been in contact with the modifying agent,e.g. by soaking in a solution containing the modifying agent at aselected concentration, the treated precursor fiber is contacted withmoisture, such as by placing the fiber under a moisture-containingatmosphere. The moisture-containing atmosphere may be one that has arelative humidity between about 50% and 100%. The precursor fibers arepreferably held under the moisture-containing atmosphere for a period oftime between about 1 hour and 60 hours.

The treated precursor fibers are then dried and pyrolyzed. The pyrolysisis advantageously conducted under an inert atmosphere. The pyrolysistemperature may be between about 500° and about 800° C.; alternatively,the pyrolysis temperature may be between about 500° and about 700° C.;alternatively, the pyrolysis temperature may be between about 500° and650° C.; alternatively, the pyrolysis temperature may be between about500° and 600° C.; alternatively, the pyrolysis temperature may bebetween about 500° and 550° C.; alternatively, the pyrolysis temperaturemay be between about 550° and about 700° C.; alternatively, thepyrolysis temperature may be between about 550° and about 650° C.alternatively the pyrolysis temperature may be between about 600° andabout 700° C.; alternatively the pyrolysis temperature may be betweenabout 600° and about 650° C. The pyrolysis temperature is typicallyreached by a process in which the temperature is slowly ramped up. Forexample, when using a pyrolysis temperature of 650° C., the pyrolysistemperature may be achieved by increasing the temperature from 50° C. to250° C. at a ramp rate of 13.3° C./min, increasing the temperature from250° C. to 635° C. at a ramp rate of 3.85° C./min, and increasing thetemperature from 635° C. to 650° C. at a ramp rate of 0.25° C./min. Oncethe pyrolysis temperature is reached, the fibers are heated at thepyrolysis temperature for a soak time, which may be a number of hours.

The polymer precursor fibers may also be bundled and pyrolyzed as abundle in order produce a large amount of modified CMS hollow fibermembranes in a single pyrolysis run. Although pyrolysis will generallybe referred to in terms of pyrolysis of a precursor fiber, it should beunderstood that any description of pyrolysis used herein is meant toinclude pyrolysis of precursor fibers that are bundled as well as thosethat are non-bundled.

Typically, heating of bundled polymer precursor fibers above the glasstransition temperature of the polymer material, such as occurs duringpyrolysis, causes the fibers to stick together. This sticking togetherof the bundled fibers reduces their desirability for use as a CMS hollowfiber membrane. By treating polymer precursor fibers with a modifyingagent, as described herein, sticking between fibers in a bundle can bereduced or eliminated. During treatment, the modifying agent reacts toform a thin film on the outer skin surfaces of the precursor fibers. Forinstance, when a precursor fiber is treated with VTMS, the treatedprecursor fiber will comprises a thin film of silicon-containingmaterial on the outer skin surface, and after pyrolysis, the modifiedCMS hollow fiber will comprise a thin film of silica on the outer skinsurface. This thin film acts as a mechanical barrier, preventing thefibers from sticking together during pyrolysis. As a result, the gasseparation properties of the asymmetric modified CMS hollow fibermembranes that undergo pyrolysis in a bundle are similar to asymmetricmodified CMS hollow fiber membranes that are not bundled duringpyrolysis.

Thus, as will be described in more detail below, it is the concentrationof the modifying agent during treatment and the pyrolysis temperaturethat most affect the gas separation properties of a modified CMS hollowfiber membrane produced from a selected precursor fiber.

Selecting the Concentration of the Modifying Agent

It has now surprisingly been found that diluting the modifying agentbefore treatment of a polymer precursor fiber results in a modified CMShollow fiber membrane having an increased permeance over a modified CMShollow fiber membrane produced by treatment of a polymer precursor witha pure modifying agent. The diluent may be any liquid that does notinterfere with the reaction of the modifying agent to form a morphologystabilizer. Suitable diluents include the C₅ or greater linear orbranched-chain aliphatic hydrocarbons. Preferred diluents, for example,include n-hexane, toluene, and n-heptane.

Comparative Example 1

Untreated Matrimid® 5218 precursor fibers were placed on a stainlesssteel wire mesh and held in place by wrapping a length of wire aroundthe mesh and fibers. The mesh support containing the fibers was thenloaded to a pyrolysis setup, such as the type that is illustrated inFIG. 14. Pyrolysis was performed under an atmosphere of ultra highpurity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting CMS fibers were tested in a single fiber module, such asthe one described by Koros et al. in U.S. Pat. No. 6,565,631, thecontents of which are hereby incorporated by reference. The CMS fibermodule was tested using a constant-volume variable pressure permeationsystem for both pure and mixed gas feeds similar to the one described byKoros et al. in U.S. Pat. No. 6,565,631. The CMS fibers were testedusing a mixed gas feed containing 50 mol % CO₂ and 50 mol % CH₄ at apressure of 150 psi (pounds per square inch). The temperature wasmaintained at 35° C.

The permeance of CO₂ through the CMS fibers was measured to be about 8to 10 GPU. The CO₂/CH₄ selectivity was determined to be about 99 to 100.

Comparative Example 2

Matrimid® 5218 precursor fibers were soaked in pure VTMS (i.e. 100 wt %VTMS) for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested in a single fiber module,such as the one described by Koros et al. in U.S. Pat. No. 6,565,631,the contents of which are hereby incorporated by reference. The CMSfiber module was tested in a constant-volume variable pressurepermeation system for both pure and mixed gas feeds similar to the onedescribed by Koros et al. in U.S. Pat. No. 6,565,631. The modified CMSfibers were tested using a mixed gas feed containing 50 mol % CO₂ and 50mol % CH₄ at a pressure of 150 psi (pounds per square inch). Thetemperature was maintained at 35° C.

The permeance of CO₂ through the modified CMS fibers was measured to beabout 35 to 40 GPU. The CO₂/CH₄ selectivity was determined to be about90 to 95.

Example 3

A solution of hexane and VTMS was prepared. The VTMS made up 75 percentby weight (75 wt %) of the solution. Matrimid® 5218 precursor fiberswere soaked in the solution for a time of about twelve hours. The fiberswere then removed from the solution and placed in a glove bag containingair at a relative humidity of 100%. After about 48 hours, the fiberswere removed and dried by heating under vacuum at 150° C. for about 12hours. The treated precursor fibers were then placed on a stainlesssteel wire mesh and held in place by wrapping a length of wire aroundthe mesh and fibers. The mesh support containing the fibers was thenloaded to a pyrolysis setup, such as the type that is illustrated inFIG. 14. Pyrolysis was performed under an atmosphere of ultra highpurity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested in a single fiber module,such as the one described by Koros et al. in U.S. Pat. No. 6,565,631,the contents of which are hereby incorporated by reference. The CMSfiber module was tested in a constant-volume variable pressurepermeation system for both pure and mixed gas feeds similar to the onedescribed by Koros et al. in U.S. Pat. No. 6,565,631. The modified CMSfibers were tested using a mixed gas feed containing 50 mol % CO₂ and 50mol % CH₄ at a pressure of 150 psi (pounds per square inch). Thetemperature was maintained at 35° C.

The permeance of CO₂ through the modified CMS fibers was measured to beabout 40 to 42 GPU. The CO₂/CH₄ selectivity was determined to be about95 to 100.

Example 4

A solution of hexane and VTMS was prepared. The VTMS made up 75 percentby weight (75 wt %) of the solution, with the hexane making up the other25 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed restricted substructure collapse.An SEM image of the CMS fibers is shown in FIG. 7.

Example 5

A solution of hexane and VTMS was prepared. The VTMS made up 50 percentby weight (50 wt %) of the solution. Matrimid® 5218 precursor fiberswere soaked in the solution for a time of about twelve hours. The fiberswere then removed from the solution and placed in a glove bag containingair at a relative humidity of 100%. After about 48 hours, the fiberswere removed and dried by heating under vacuum at 150° C. for about 12hours. The treated precursor fibers were then placed on a stainlesssteel wire mesh and held in place by wrapping a length of wire aroundthe mesh and fibers. The mesh support containing the fibers was thenloaded to a pyrolysis setup, such as the type that is illustrated inFIG. 14. Pyrolysis was performed under an atmosphere of ultra highpurity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 45 to 48 GPU. The CO₂/CH₄ selectivity was determined to be about90 to 95.

Example 6

A solution of hexane and VTMS was prepared. The VTMS made up 50 percentby weight (50 wt %) of the solution, with the hexane making up the other50 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed restricted substructure collapse.An SEM image of the CMS fibers is shown in FIG. 8.

Example 7

A solution of hexane and VTMS was prepared. The VTMS made up 25 percentby weight (25 wt %) of the solution. Matrimid® 5218 precursor fiberswere soaked in the solution for a time of about twelve hours. The fiberswere then removed from the solution and placed in a glove bag containingair at a relative humidity of 100%. After about 48 hours, the fiberswere removed and dried by heating under vacuum at 150° C. for about 12hours. The treated precursor fibers were then placed on a stainlesssteel wire mesh and held in place by wrapping a length of wire aroundthe mesh and fibers. The mesh support containing the fibers was thenloaded to a pyrolysis setup, such as the type that is illustrated inFIG. 14. Pyrolysis was performed under an atmosphere of ultra highpurity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 50 to 55 GPU. The CO₂/CH₄ selectivity was determined to be about88 to 91.

Example 8

A solution of hexane and VTMS was prepared. The VTMS made up 25 percentby weight (25 wt %) of the solution, with the hexane making up the other75 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed restricted substructure collapse.An SEM image of the CMS fibers is shown in FIG. 9.

Example 9

A solution of hexane and VTMS was prepared. The VTMS made up 10 percentby weight (10 wt %) of the solution. Matrimid® 5218 precursor fiberswere soaked in the solution for a time of about twelve hours. The fiberswere then removed from the solution and placed in a glove bag containingair at a relative humidity of 100%. After about 48 hours, the fiberswere removed and dried by heating under vacuum at 150° C. for about 12hours. The treated precursor fibers were then placed on a stainlesssteel wire mesh and held in place by wrapping a length of wire aroundthe mesh and fibers. The mesh support containing the fibers was thenloaded to a pyrolysis setup, such as the type that is illustrated inFIG. 14. Pyrolysis was performed under an atmosphere of ultra highpurity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 65 to 70 GPU. The CO₂/CH₄ selectivity was determined to be about85 to 90.

Example 10

A solution of hexane and VTMS was prepared. The VTMS made up 10 percentby weight (10 wt %) of the solution, with the hexane making up the other90 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed restricted substructure collapse.An SEM image of the CMS fibers is shown in FIG. 8.

Example 11

A solution of hexane and VTMS was prepared. The VTMS made up 5 percentby weight (5 wt %) of the solution. Matrimid® 5218 precursor fibers weresoaked in the solution for a time of about twelve hours. The fibers werethen removed from the solution and placed in a glove bag containing airat a relative humidity of 100%. After about 48 hours, the fibers wereremoved and dried by heating under vacuum at 150° C. for about 12 hours.The treated precursor fibers were then placed on a stainless steel wiremesh and held in place by wrapping a length of wire around the mesh andfibers. The mesh support containing the fibers was then loaded to apyrolysis setup, such as the type that is illustrated in FIG. 14.Pyrolysis was performed under an atmosphere of ultra high purity argon(99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 18 to 20 GPU. The CO₂/CH₄ selectivity was determined to be about99 to 100.

Example 12

A solution of hexane and VTMS was prepared. The VTMS made up 5 percentby weight (5 wt %) of the solution, with the hexane making up the other95 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed partial substructure collapse. AnSEM image of the CMS fibers is shown in FIG. 9.

Example 13

A solution of hexane and VTMS was prepared. The VTMS made up 1 percentby weight (1 wt %) of the solution. Matrimid® 5218 precursor fibers weresoaked in the solution for a time of about twelve hours. The fibers werethen removed from the solution and placed in a glove bag containing airat a relative humidity of 100%. After about 48 hours, the fibers wereremoved and dried by heating under vacuum at 150° C. for about 12 hours.The treated precursor fibers were then placed on a stainless steel wiremesh and held in place by wrapping a length of wire around the mesh andfibers. The mesh support containing the fibers was then loaded to apyrolysis setup, such as the type that is illustrated in FIG. 14.Pyrolysis was performed under an atmosphere of ultra high purity argon(99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 635° C. at a ramp rate of 3.85° C./min-   3. 635° C. to 650° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 650° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 10 to 12 GPU. The CO₂/CH₄ selectivity was determined to be about99 to 100.

Example 14

A solution of hexane and VTMS was prepared. The VTMS made up 1 percentby weight (1 wt %) of the solution, with the hexane making up the other99 weight percent. Matrimid® 5218 precursor fibers were soaked in thesolution for a time of about twelve hours. The fibers were then removedfrom the solution and placed in a glove bag containing air at a relativehumidity of 100%. After about 48 hours, the fibers were removed anddried by heating under vacuum at 150° C. for about 12 hours. The treatedprecursor fibers were then placed on a stainless steel wire mesh andheld in place by wrapping a length of wire around the mesh and fibers.The mesh support containing the fibers was then loaded to a pyrolysissetup, such as the type that is illustrated in FIG. 14. Pyrolysis wasperformed under an atmosphere of ultra high purity argon (99.9% pure) asfollows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were analyzed by scanning electronmicroscope (SEM). SEM analysis showed collapsed substructure morphology.An SEM image of the CMS fibers is shown in FIG. 9.

The testing results of the above Examples are summarized in Table 1.

TABLE 1 P(CO₂) [GPU] CO₂/CH₄ CMS untreated Matrimid ® 5218  8-10  99-100(Pyrolysis Temperature 650° C.) CMS Treated (100% VTMS) Matrimid ® 521835-40 90-95 (Pyrolysis Temperature 650° C.) CMS Treated (75% VTMS)Matrimid ® 5218 40-42  95-100 (Pyrolysis Temperature 650° C.) CMSTreated (50% VTMS) Matrimid ® 5218 45-48 90-95 (Pyrolysis Temperature650° C.) CMS Treated (25% VTMS) Matrimid ® 5218 50-55 88-91 (PyrolysisTemperature 650° C.) CMS Treated (10% VTMS) Matrimid ® 5218 65-70 85-90(Pyrolysis Temperature 650° C.) CMS Treated (5% VTMS) Matrimid ® 521818-20  99-100 (Pyrolysis Temperature 650° C.) CMS Treated (1% VTMS)Matrimid ® 5218 10-12  99-100 (Pyrolysis Temperature 650° C.)

As demonstrated by the above Examples, a polymer precursor fiber that iscontacted with a solution comprising a modifying agent that is presentin an amount that is less than 100% of the solution surprisinglyproduces a modified CMS fiber membrane having increased gas permeancewhen compared against a polymer precursor fiber that is contacted with100% pure modifying agent. Rather, it has presently been found thatuntil the concentration of modifying agent reaches a point at which itappears to no longer be effective at significantly restricting thecollapse of the substructure pores during pyrolysis, the gas permeanceof the resulting modified CMS fiber membrane actually increases inresponse to a decrease in the concentration of the modifying agent inthe treatment solution. For VTMS, the point at which the solutionappears to no longer be effective at significantly restrictingsubstructure collapse appears to occur at a concentration between about1% and about 5% by weight. Of the tested solutions, the solution thatproduced a modified CMS fiber membrane having the highest gas permeancecontained modifying agent at a concentration of about 10% by weight.

Various embodiments of the present invention are directed towardcontacting a polymer precursor fiber with a solution comprisingmodifying agent at a concentration between about 1%) and about 95% byweight; alternatively between about 1%) and about 90%; alternativelybetween about 1% and about 80%; alternatively between about 1% and about75%; alternatively between about 1%) and about 50%; alternativelybetween about 1%) and about 25%; alternatively between about 1%) andabout 15%; alternatively between about 1%) and about 12%; alternativelybetween about 5% and about 90% by weight; alternatively between about 5%and about 80%; alternatively between about 5% and about 75%;alternatively between about 5% and about 50%; alternatively betweenabout 5% and about 25%; alternatively between about 5% and about 15%;alternatively between about 5% and about 12%; alternatively betweenabout 5% and about 10%; alternatively between about 8% and about 90% byweight; alternatively between about 8% and about 80%; alternativelybetween about 8% and about 75%; alternatively between about 8% and about50%; alternatively between about 8% and about 25%; alternatively betweenabout 8% and about 15%; alternatively between about 8% and about 12%;alternatively between about 10% and about 90% by weight; alternativelybetween about 10% and about 80%; alternatively between about 10% andabout 75%; alternatively between about 10% and about 50%; alternativelybetween about 10% and about 25%; alternatively between about 10% andabout 15%. The above percentages represent the percent by weight ofmodifying agent in the contacting solution.

Treatment of a polymer precursor fiber with a modifying agent alsoresults in the formation of a residual film on the outer skin layer ofthe asymmetric modified CMS hollow fiber membrane. Specifically, it isbelieved that in addition to undergoing a sol-gel reaction processwithin the pores of the precursor fiber, the modifying agent undergoes asimilar reaction to form a film on the outer skin layer of theasymmetric modified CMS hollow fiber. Importantly, this film operates toprevent sticking of the precursor fibers together when they are heatedabove the glass transition temperature of the polymer, such as duringpyrolysis.

This film also operates, however, to inhibit the flow of gas through thepores that are present on the outer skin layer of the fiber, decreasingthe permeance (and effectiveness) of the asymmetric modified CMS hollowfiber membrane. The formation of a film by the sol-gel reaction ofvinyltrimethoxysilane (VTMS), a preferred modifying agent of the presentinvention, is illustrated in FIG. 3. Accordingly, by reducing theconcentration of modifying agent, the formation of the residual film islimited, leading to an increase in gas permeance of the resulting CMShollow fiber membrane. Without being bound by theory, this effect isbelieved to explain the surprising result that lowering theconcentration of modifying agent in a contacting solution results in amodified CMS hollow fiber membrane having an increased gas permeanceproperty.

Thus, it is an object of the present invention to treat a precursorfiber with a modifying agent, wherein the modifying agent is present ata concentration that is effective to limit the formation of the residualfilm, substantially minimizing the thickness of the film. Preferably,the concentration of the modifying agent is selected in an amount thatis both effective, i.e. sufficiently high to significantly restrictsubstructure collapse and effective, i.e. sufficiently low to limit theformation of a residual film on the outer skin layer of the asymmetricmodified CMS hollow fiber. In this way, an asymmetric modified CMShollow fiber membrane having both an improved permeance property andbeneficial non-stick properties may be prepared.

In various embodiments, the concentration of the modifying agent in thesolution is selected to obtain an asymmetric modified CMS hollow fibermembrane having a gas permeance that is at least a 300% increase over anequivalent asymmetric CMS hollow fiber membrane that is not subjected totreatment with the modifying agent. In various embodiments, theconcentration of the modifying agent in the solution is selected toobtain an asymmetric modified CMS hollow fiber membrane having a gaspermeance that is at least a 400% increase over an equivalent asymmetricCMS hollow fiber membrane that is not subjected to treatment with themodifying agent. In various embodiments, the concentration of themodifying agent in the solution is selected to obtain an asymmetricmodified CMS hollow fiber membrane having a gas permeance that is atleast a 500% increase over an equivalent asymmetric CMS hollow fibermembrane that is not subjected to treatment with the modifying agent. Invarious embodiments, the concentration of the modifying agent in thesolution is selected to obtain an asymmetric modified CMS hollow fibermembrane having a gas permeance that is at least a 600% increase over anequivalent asymmetric CMS hollow fiber membrane that is not subjected totreatment with the modifying agent.

In various embodiments, the concentration of the modifying agent in thesolution is selected to obtain an asymmetric modified CMS hollow fibermembrane having a desired combination of gas permeance and selectivityproperties. For example, the concentration of the modifying agent in thesolution may be selected to obtain an asymmetric modified CMS hollowfiber membrane having properties that are useful for the separation ofCO₂ and CH₄ within a gas stream comprising any number of additionalconstituents. Alternatively, the concentration of the modifying agent inthe solution may be selected to obtain an asymmetric modified CMS hollowfiber membrane that is useful for the separation of H₂S and CH₄ within agas stream comprising any number of additional constituents.Alternatively, the concentration of the modifying agent in the solutionmay be selected to obtain an asymmetric modified CMS hollow fibermembrane that is useful for the separation of a mixture of CO₂ and H₂S(CO₂/H₂S) from CH₄ within a gas stream comprising any number ofadditional constituents. Alternatively, the concentration of themodifying agent in the solution may be selected to obtain an asymmetricmodified CMS hollow fiber membrane that is useful for the separation ofCO₂ and N₂ within a gas stream comprising any number of additionalconstituents. Alternatively, the concentration of the modifying agent inthe solution may be selected to obtain an asymmetric modified CMS hollowfiber membrane that is useful for the separation of O₂ and N₂ within agas stream comprising any number of additional constituents.Alternatively, the concentration of the modifying agent in the solutionmay be selected to obtain an asymmetric modified CMS hollow fibermembrane that is useful for the separation of N₂ and CH₄ within a gasstream comprising any number of additional constituents. Alternatively,the concentration of the modifying agent in the solution may be selectedto obtain an asymmetric modified CMS hollow fiber membrane that isuseful for the separation of He and CH₄ within a gas stream comprisingany number of additional constituents. Alternatively, the concentrationof the modifying agent in the solution may be selected to obtain anasymmetric modified CMS hollow fiber membrane that is useful for theseparation of H₂ and CH₄ within a gas stream comprising any number ofadditional constituents. Alternatively, the concentration of themodifying agent in the solution may be selected to obtain an asymmetricmodified CMS hollow fiber membrane that is useful for the separation ofH₂ and C₂H₄ within a gas stream comprising any number of additionalconstituents. Alternatively, the concentration of the modifying agent inthe solution may be selected to obtain an asymmetric modified CMS hollowfiber membrane that is useful for the separation of olefins fromparaffins, such as the separation of ethylene and ethane or propyleneand propane within a gas stream comprising any number of additionalconstituents. The concentration of the modifying agent in the solutionmay also be selected to obtain an asymmetric modified CMS hollow fibermembrane that is useful for the separation of a mixture of olefins froma mixture of paraffins, such as a mixture of ethylene and propylene(ethylene/propylene) from a mixture of ethane and propane(ethane/propane) within a gas stream comprising any number of additionalconstituents.

In one embodiment, the concentration of the modifying agent in thesolution may be selected to obtain an asymmetric modified CMS hollowfiber membrane that is useful for the separation of acid gases, such asCO₂ and H₂S, from a gas stream that contains or is rich in hydrocarbons,such as a natural gas stream.

In various embodiments, the concentration of the modifying agent in thesolution is selected to obtain an asymmetric modified CMS hollow fibermembrane having desirable permeance and selectivity properties, such asmay be determined by testing the asymmetric modified CMS hollow fibermembrane in a single fiber module using a constant-volume variablepressure permeation system such as the one described by Koros et al. inU.S. Pat. No. 6,565,631. For example, where the concentration of themodifying agent in the solution is selected to obtain an asymmetricmodified CMS hollow fiber membrane having properties that are desirablefor the separation of CO₂ and CH₄, the concentration of the modifyingagent in the solution may be selected to obtain an asymmetric modifiedCMS hollow fiber membrane having a CO₂ permeance of at least 50 GPU anda CO₂/CH₄ selectivity of at least 60 when subjected to a mixed feedcontaining 50 mol % CO₂ and 50 mol % CH₄ at 150 psi and 35° C.Alternatively, the concentration of the modifying agent in the solutionmay be selected to obtain an asymmetric CMS hollow fiber membrane havinga CO₂ permeance of at least 60 GPU and a CO₂/CH₄ selectivity of at least80 when subjected to a mixed feed containing 50 mol % CO₂ and 50 mol %CH₄ at 150 psi and 35° C.

Although the above examples show the manner in which the concentrationof modifying agent in solution may be selected to obtain an asymmetricmodified CMS hollow fiber membrane having properties that are desirablefor the separation of CO₂ and CH₄, it will be understood by a person ofordinary skill in the art that by testing asymmetric modified CMS hollowfiber membranes prepared using varying concentrations of modifying agentin the separation of a different gas stream, one may readily determinethe concentration (or range of concentrations) of modifying agent in thesolution that produces an asymmetric modified CMS hollow fiber membranethat is particularly desirable for separation of any gas stream.

Control of the Concentration of Modifying Agent and the PyrolysisTemperature

In various embodiments, the pyrolysis temperature is also selected toobtain an asymmetric modified CMS hollow fiber membrane having a desiredcombination of gas permeance and selectivity properties. The pyrolysistemperature at which a desired combination of properties is achievedwill vary depending on the polymer precursor that is used. By carefulcontrol of both the concentration of modifying agent used in thepre-pyrolysis treatment and the temperature of the pyrolysis, anasymmetric modified CMS hollow fiber membrane having certain desired gasseparation properties may be prepared.

To demonstrate how the gas separation properties of an asymmetricmodified CMS hollow fiber membrane may be adjusted by control of boththe concentration of modifying agent in the treatment step and thepyrolysis temperature, Matrimid® 5218 fibers were treated with asolution containing 10% by weight VTMS as described in Examples 10 and11. The precursor fibers were then subjected to pyrolysis at varioustemperatures in order to determine the most suitable pyrolysistemperature for a selected polymer precursor that was treated with asolution comprising modifying agent at a concentration shown to yield animproved gas permeance. For comparison, untreated Matrimid® 5218 fibersand Matrimid® 5218 fibers treated with pure (i.e. 100%) VTMS weresubjected to pyrolysis over a similar range of temperatures. The testingis described in more detail below.

Example 15

Matrimid® 5218 precursor fibers treated with a solution of 10 wt % VTMSas in Examples 9 and 10 were subjected to pyrolysis under an atmosphereof ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 190 to 195 GPU. The CO₂/CH₄ selectivity was determined to be about15 to 20.

Example 16

Matrimid® 5218 precursor fibers treated with a solution of 10 wt % VTMSas in Examples 9 and 10 were subjected to pyrolysis under an atmosphereof ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 585° C. at a ramp rate of 3.85° C./min-   3. 585° C. to 600° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 600° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 170 to 180 GPU. The CO₂/CH₄ selectivity was determined to be about38 to 40.

Example 17

Matrimid® 5218 precursor fibers treated with a solution of 10 wt % VTMSas in Examples 9 and 10 were subjected to pyrolysis under an atmosphereof ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 610° C. at a ramp rate of 3.85° C./min-   3. 610° C. to 625° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 625° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 70 to 75 GPU. The CO₂/CH₄ selectivity was determined to be about75 to 80.

Comparative Example 3

Untreated Matrimid® 5218 precursor fibers were subjected to pyrolysisunder an atmosphere of ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting CMS fibers were tested as described in Example 3. Thepermeance of CO₂ through the CMS fibers was measured to be about 20 to30 GPU. The CO₂/CH₄ selectivity was determined to be about 30 to 40.

Comparative Example 4

Matrimid® 5218 precursor fibers treated with a solution of pure VTMS(100 wt %) as in Comparative Example 2 were subjected to pyrolysis underan atmosphere of ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 535° C. at a ramp rate of 3.85° C./min-   3. 535° C. to 550° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 550° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 100 to 120 GPU. The CO₂/CH₄ selectivity was determined to be about20 to 25.

Comparative Example 5

Matrimid® 5218 precursor fibers treated with a solution of pure VTMS(100 wt %) as in Comparative Example 2 were subjected to pyrolysis underan atmosphere of ultra high purity argon (99.9% pure) as follows:

-   1. 50° C. to 250° C. at a ramp rate of 13.3° C./min-   2. 250° C. to 585° C. at a ramp rate of 3.85° C./min-   3. 585° C. to 600° C. at a ramp rate of 0.25° C./min-   4. Soak for 2 hours at 600° C.

The resulting modified CMS fibers were tested as described in Example 3.The permeance of CO₂ through the modified CMS fibers was measured to beabout 60 to 65 GPU. The CO₂/CH₄ selectivity was determined to be about30 to 35.

The results of the testing are summarized in Table 2. Additionally, acomparison of the results at two different temperatures (550° C. and650° C.) is shown in FIG. 13.

From these results, it can be seen that in order to obtain an asymmetricmodified CMS hollow fiber membrane having superior performance from aMatrimid® 5218 precursor fiber treated with a VTMS modifying agent, theprecursor fiber may be contacted with a treatment solution containingabout 10 wt % VTMS and then pyrolyzed at a temperature between about600° C. and 650° C. Using the techniques described herein, the suitableranges of (a) the concentration of modifying agent in the treatmentsolution and (b) the pyrolysis temperature could similarly be determinedfor any precursor fiber. For many precursor fibers, including forexample polyimide precursor fibers, desirable treatment concentrationsand pyrolysis temperatures are expected to be similar to thosedemonstrated for the Matrimid® 5218 precursor fiber.

TABLE 2 P(CO₂) [GPU] CO₂/CH₄ CMS untreated Matrimid ® 5218 20-30 30-40(Pyrolysis Temperature 550° C.) CMS untreated Matrimid ® 5218  8-10 99-100 (Pyrolysis Temperature 650° C.) CMS Treated(100% VTMS)Matrimid ® 5218 100-120 20-25 (Pyrolysis Temperature 550° C.) CMSTreated(100% VTMS) Matrimid ® 5218 60-65 30-35 (Pyrolysis Temperature600° C.) CMS Treated(100% VTMS) Matrimid ® 5218 35-40 90-95 (PyrolysisTemperature 650° C.) CMS Treated(10% VTMS) Matrimid ® 5218 190-195 15-20(Pyrolysis Temperature 550° C.) CMS Treated(10% VTMS) Matrimid ® 5218170-180 38-40 (Pyrolysis Temperature 600° C.) CMS Treated(10% VTMS)Matrimid ® 5218 70-75 75-80 (Pyrolysis Temperature 625° C.) CMSTreated(10% VTMS) Matrimid ® 5218 65-70 85-90 (Pyrolysis Temperature650° C.)

Although the above examples show the manner in which the concentrationof modifying agent and pyrolysis temperature may together be selected toobtain an asymmetric modified CMS hollow fiber membrane havingproperties that are desirable for the separation of CO₂ and CH₄, it willbe understood by a person of ordinary skill in the art that by testingasymmetric modified CMS hollow fiber membranes prepared using varyingconcentrations of modifying agent and pyrolysis temperatures for theseparation of any gas stream, one may readily determine theconcentration (or range of concentrations) of modifying agent and thepyrolysis temperature that produces an asymmetric modified CMS hollowfiber membrane that is particularly desirable for separation of any gasstream.

Enhanced Solvent Exchange

In various embodiments, treatment of the polymer precursor fiber may becoupled with the solvent exchange process. After the precursor fibersare formed, such as by the dry-jet, wet-quench method, the fibers aresubjected to a process known as solvent exchange. In order to maintainthe porosity of the fibers through drying, it is necessary to remove thewater contained within the pores of the membrane. Accordingly, thesolvent exchange process replaces the water that is present in theporous substructure of the fiber with an organic compound having a lowsurface tension. The precursor fiber is subjected to the solventexchange process for a time that is effective to allow the organiccompound to replace the water that is present in the pores of the fiber.

Solvent exchange preferably involves two or more steps, with each stepusing a different solvent exchange material. By way of example, aconventional solvent exchange process includes removing the water in themembrane with a first solvent and then replacing the alcohol with asecond solvent. The first step uses one or more solvent exchangematerials comprising a water-miscible alcohol that is sufficiently inertto the polymer. Any compound that is effective for replacing water inthe membrane is contemplated for use as a first solvent. The aliphaticalcohols with 1-3 carbon atoms, i.e. methanol, ethanol, propanol,isopropanol, and combinations of the above, are particularly effectiveas a first solvent exchange material.

The second step is effective to replace the alcohol with one or morevolatile organic compounds having a low surface tension. Any organicsolvent that has a sufficiently low surface tension to prevent damage tothe membrane pores during heating is contemplated for use as a secondsolvent. Among the organic compounds that are particularly useful as asecond solvent exchange material are the C₅ or greater linear orbranched-chain aliphatic alkanes. Toluene has also been suggested foruse as a second solvent. N-hexane has been found to be a particularlysuitable organic compound for use as the second solvent exchangematerial. Advantageously, the first and second solvent exchangematerials should be sufficiently nonreactive with the membrane so as toprevent any significant degradation of membrane properties.

Although the process described in this exemplary embodiment includesonly two steps, the solvent exchange process may involve any number ofsteps and any number of solvents. A solvent exchange process may makeuse of any number of solvent exchange materials, with the solventexchange material of each subsequent step being effective to replace thesolvent exchange material of the preceding step.

It has presently been found that by using a solvent exchange materialthat includes an amount of a modifying agent, an asymmetric modified CMShollow fiber membrane having enhanced gas permeance may be prepared.Since treatment of a precursor fiber with a modifying agent in order torestrict the substructure collapse may be performed in association withthe solvent exchange process, a modified CMS hollow fiber membranehaving enhanced gas permeance may be prepared without the need for anadditional process step beyond those which are typically performed inthe preparation of an asymmetric CMS hollow fiber membrane. The processof treating the precursor fiber with a modifying agent, as describedherein, as part of the solvent exchange step may be referred to as anenhanced solvent exchange process.

Accordingly, in various embodiments, a modifying agent is added to thesecond solvent in the solvent exchange process. For instance, themodifying agent is added to an organic solvent that has a sufficientlylow surface tension to maintain the membrane pores during drying. In onesuitable embodiment, the organic solvent is n-hexane. The concentrationof the modifying agent in the organic solvent may be selected asdescribed in this specification.

In an enhanced solvent exchange process, the precursor fiber is soakedin a solution comprising an organic compound, such as n-hexane, and amodifying agent, such as VTMS, for a period of time that is effective toallow the modifying agent to react with a portion of the water in thepores of the fiber and the organic solvent to replace another portion ofthe water in the pores of the fiber. In this way, the precursor fibersacquire the benefits of treatment with the modifying agent withoutlosing the benefits of the conventional solvent exchange process.

The ability of the modifying agent to penetrate the outer skin of theasymmetric precursor fiber renders it particularly attractive fortreatment of the fiber during the solvent exchange process. In acommercial process, a precursor fiber is often conveyed through thesolvent exchange material in a continuous manner. Accordingly, the endsof the fiber rarely, if ever, come into contact with the solventexchange material. Thus, for effective treatment during the commercialmanufacture of an asymmetric CMS hollow fiber membrane, the modifyingagent reaches the substructure of the precursor fiber through the outerskin of the fiber.

Asymmetric Modified CMS Hollow Fiber Membranes

Various embodiments of the present invention are directed to anasymmetric CMS hollow fiber membrane having a morphology stabilizerwithin at least one of its pores. In one desirable embodiment, the poreis a substructure pore and the morphology stabilizer is itself porous.

Example 18

Elemental analysis was performed on the following:

-   a. untreated Matrimid® 5218 precursor fibers,-   b. Matrimid® 5218 precursor fibers treated with pure (100%) VTMS in    accordance with Examples 1 and 2,-   c. asymmetric CMS hollow fibers prepared by pyrolyzing untreated    Matrimid® 5218 precursor fibers at 550° C. and 650° C., and-   d. asymmetric modified CMS hollow fibers prepared by pyrolyzing    Matrimid® 5218 precursor fibers treated with pure (100%) VTMS in    accordance with Example 1 and 2 at 550° C. and 650° C.-   e. asymmetric modified CMS hollow fibers prepared by pyrolyzing    Matrimid® 5218 precursor fibers treated with a solution comprising    10% by weight VTMS, as set forth in Examples 9 and 10.

The elemental analysis was obtained from ALS Environmental Lab inTucson, Ariz. The elemental analysis comprised a number of techniques toidentify and measure the amount of carbon, hydrogen, nitrogen, oxygen,and silicon present in each fiber sample.

The carbon, hydrogen, and nitrogen contents were determined using amicro CHN Analysis (ASTM D5373/D5291) method. The instrument used inthis method was a Perkin Elmer 2400 Series II CHN Analyzer. With thisinstrument, samples were combusted at 935° C., followed by a secondarycombustion through the furnace at 840° C. for further oxidation andparticulate removal. The gas derived from the combustion is transferredby a carrier gas, homogenized and purged through an IR detector. Thisdetector measures carbon by CO₂ gas and hydrogen from H₂O. The nitrogenis detected by thermo conductivity in which the NO₂ gas from theresulting combustate is measured as nitrogen. The CHN results were thenreported as a weight percent. The molar percentages of carbon, hydrogenand nitrogen were then calculated from the measured weight percentages.The samples were prepared as follows. An amount of sample, derived fromthe sample matrix, was weighed out on a micro-balance having a 0.0001 mgcapability. Each sample was then placed into a pre-weighed, combustibletin capsule and dropped into the furnace of the instrument for analysis.The instrument was calibrated for the specific matrix of the sample andthe capsule used.

The oxygen content was determined by an Oxygen Analysis (ASTM D5373,modified) method. The instrument used in this method was a LECO TruSpecOxygen Analyzer. Each sample was put into a capsule and weighed out on amicro balance with 0.001 mg capability. The capsule was then droppedinto the furnace which operates at 1300° C. In a reduction tube, brokenup O₂ is combined with carbon black in the furnace. All CO_(x)components are flowed through copper oxide and are converted to CO₂.This resulting gas is analyzed for oxygen by IR detection. Theinstrument was calibrated for the specific matrix of the sample and thecapsule used in combustion. The results were reported as a weightpercent of oxygen in the sample. The molar percentages of oxygen werethen calculated from the measured weight percentages.

The content of silicon in the modified CMS hollow fiber membranes wasdetermined by a method utilizing total dissolution. Specifically, theanalysis was performed by the ICP-OES technique. This technique involvesdigestion of a sample with acids (such as HCl, HNO₃, HF) in a microwave,complexing with boric acid to neutralize HF, and bringing up to a finalvolume with nanopure water. The results were reported as a weightpercent of silicon in the sample. The molar percentages of silicon werethen calculated from the measured weight percentages.

The results of the elemental analyses are set forth in Tables 3, 4 and5.

TABLE 3 Carbon Hydrogen Nitrogen Oxygen Silicon (wt %) (wt %) (wt %) (wt%) (wt %) Untreated Matrimid ® 75.39 4.42 5.32 14.50 — 5218 PrecursorFiber Treated (100%) VTMS 73.20 4.85 4.53 13.19 4.23 Matrimid ® 5218Precursor FiberElemental analysis demonstrates that the Matrimid® 5218 precursor fibersthat were treated by soaking in a liquid consisting of pure VTMScontained about 4 wt % silicon. To ensure that the measured silicon isattributable to the chain-like network of siloxane bridges that areformed by the reaction of the VTMS and the moisture that resides withinthe pores of the asymmetric precursor hollow fiber, untreated Matrimid®5218 precursor fibers were also subjected to elemental analysis. Theuntreated Matrimid® 5218 precursor fibers were found to contain nomeasurable amount of silicon. Accordingly, the silicon found byelemental analysis of the treated Matrimid® 5218 precursor fibers canserve to indicate the amount of chain-linked modifying agent condensatethat resides in a precursor fiber after treatment.

TABLE 4 Carbon Hydrogen Nitrogen Oxygen Silicon (wt %) (wt %) (wt %) (wt%) (wt %) Untreated CMS untreated Matrimid ® 5218 87.37 3.09 4.8 4.8 —(Pyrolysis Temperature 550° C.) CMS untreated Matrimid ® 5218 91.19 1.273.78 2.67 — (Pyrolysis Temperature 650° C.) Treated (100% VTMS) CMSTreated(100% VTMS) Matrimid ® 76.06 3.24 3.96 5.17 11.56 5218 (PyrolysisTemperature 550° C.) CMS Treated(100% VTMS) Matrimid ® 76.34 2.35 2.692.99 15.63 5218 (Pyrolysis Temperature 650° C.) Treated (10% VTMS) CMSTreated(10% VTMS) Matrimid ® 85.60 3.36 3.91 2.95 4.19 5218 (PyrolysisTemperature 550° C.) CMS Treated(10% VTMS) Matrimid ® 87.64 2.73 3.351.98 4.30 5218 (Pyrolysis Temperature 600° C.) CMS Treated(10% VTMS)Matrimid ® 86.05 2.47 3.09 2.57 5.82 5218 (Pyrolysis Temperature 650°C.)

Elemental analysis was also performed on asymmetric modified CMS hollowfibers that were prepared by treating Matrimid® 5218 precursor fiberswith pure VTMS and then pyrolyzing the treated fibers. The resultsindicate that the modified CMS hollow fibers contain between about 11 wt% silicon and about 16 wt % silicon (between about 3 and about 6 mol %silicon), depending on the pyrolysis temperature (which ranged from 550°C. to 650° C.). To ensure that the measured silicon was attributable tothe morphology stabilizer that is present after pyrolysis, CMS hollowfibers prepared by pyrolyzing untreated Matrimid® 5218 precursor fibers(using the same pyrolysis temperatures) were also subjected to elementalanalysis. The CMS hollow fibers prepared from untreated Matrimid® 5218precursor fibers were found to contain no measurable amount of silicon.Accordingly, the weight percent of silicon found by elemental analysisof modified CMS hollow fibers can serve to indicate the amount ofmorphology stabilizer that resides in an asymmetric modified CMS hollowfiber membrane.

Next, elemental analysis was performed on asymmetric modified CMS hollowfibers that were prepared by treating Matrimid® 5218 precursor fiberswith a solution containing 10% by weight VTMS, and then pyrolyzing thetreated fibers at different pyrolysis temperatures ranging from 550° C.to 650° C. The modified CMS hollow fiber membranes were found to containbetween about 4% by weight silicon and about 6% by weight silicon(between about 1 and about 2 mol % silicon).

TABLE 5 Carbon Hydrogen Nitrogen Oxygen Silicon (mol %) (mol %) (mol %)(mol %) (mol %) Untreated CMS untreated 66.11 28.06 3.11 2.72 —Matrimid ® (Pyrolysis Temperature 550° C.) CMS untreated 81.66 13.652.90 1.79 — Matrimid ® (Pyrolysis Temperature 650° C.) Treated (100%VTMS) CMS Treated(100% 56.17 35.55 2.29 2.36 3.62 VTMS) Matrimid ®(Pyrolysis Temperature 550° C.) CMS Treated(100% 65.96 24.34 1.99 1.945.77 VTMS) Matrimid ® (Pyrolysis Temperature 650° C.) Treated (10% VTMS)CMS Treated(10% 64.21 30.28 2.51 1.66 1.34 VTMS) Matrimid ® (PyrolysisTemperature 550° C.) CMS Treated(10% 69.22 25.88 2.27 1.17 1.45 VTMS)Matrimid ® (Pyrolysis Temperature 600° C.) CMS Treated(10% 70.08 24.172.16 1.57 2.03 VTMS) Matrimid ® (Pyrolysis Temperature 650° C.)

In one embodiment, the asymmetric modified CMS hollow fiber membranecomprises an amount of morphology stabilizer that can be determined byelemental analysis, such that the asymmetric modified CMS hollow fibermembrane comprises a desired molar percentage of an element whose onlysignificant presence in the fiber is attributable to treatment with amodifying agent, i.e. an indicating element. The indicating elementcomprises, for example, silicon and/or the metal element that forms thehead of the silane and/or metal alkoxide modifying agents. For example,the asymmetric modified CMS hollow fiber membrane may comprise betweenabout 0.1 mol % and about 10 mol % of the indicating element;alternatively between about 0.1 mol % and about 8 mol %; alternativelybetween about 0.1 mol % and about 7 mol %; alternatively between about0.1 mol % and about 6 mol %; alternatively between about 0.1 mol % andabout 5 mol %; alternatively between about 0.1 mol % and about 4 mol %;alternatively between about 0.1 mol % and about 3 mol %; alternativelybetween about 0.1 mol % and about 2 mol %; alternatively between about0.5 mol % and about 10 mol %; alternatively between about 0.5 mol % andabout 8 mol %; alternatively between about 0.5 mol % and about 7 mol %;alternatively between about 0.5 mol % and about 6 mol %; alternativelybetween about 0.5 mol % and about 5 mol %; alternatively between about0.5 mol % and about 4 mol %; alternatively between about 0.5 mol % andabout 3 mol %; alternatively between about 0.5 mol % and about 2 mol %;alternatively between about 0.75 mol % and about 10 mol %; alternativelybetween about 0.75 mol % and about 8 mol %; alternatively between about0.75 mol % and about 7 mol %; alternatively between about 0.75 mol % andabout 6 mol %; alternatively between about 0.75 mol % and about 5 mol %;alternatively between about 0.75 mol % and about 4 mol %; alternativelybetween about 0.75 mol % and about 3 mol %; alternatively between about0.75 mol % and about 2 mol %; alternatively between about 1 mol % andabout 10 mol %; alternatively between about 1 mol % and about 8 mol %;alternatively between about 1 mol % and about 7 mol %; alternativelybetween about 1 mol % and about 6 mol %; alternatively between about 1mol % and about 5 mol %; alternatively between about 1 mol % and about 4mol %; alternatively between about 1 mol % and about 3 mol %;alternatively between about 1 mol % and about 2 mol %.

Advantageously, the asymmetric modified CMS hollow fiber membrane maycomprise an amount of morphology stabilizer such that the asymmetricmodified CMS hollow fiber membrane comprises a desired weight percentageof indicating element. For example, in embodiments where the morphologystabilizer comprises a silicon-containing compound, the asymmetricmodified CMS hollow fiber membrane may comprise between about 0.1 wt %and about 20 wt % silicon; alternatively between about 0.1 wt % andabout 15 wt %; alternatively between about 0.1 wt % and about 10 wt %;alternatively between about 0.1 wt % and about 8 wt %; alternativelybetween about 0.1 wt % and about 6 wt %; alternatively between about 0.1wt % and about 5 wt %; alternatively between about 0.5 wt % and about 20wt % silicon; alternatively between about 0.5 wt % and about 15 wt %;alternatively between about 0.5 wt % and about 10 wt %; alternativelybetween about 0.5 wt % and about 8 wt %; alternatively between about 0.5wt % and about 6 wt %; alternatively between about 0.5 wt % and about 5wt %; alternatively between about 1 wt % and about 20 wt % silicon;alternatively between about 1 wt % and about 15 wt %; alternativelybetween about 1 wt % and about 10 wt %; alternatively between about 1 wt% and about 8 wt %; alternatively between about 1 wt % and about 6 wt %;alternatively between about 1 wt % and about 5 wt %; alternativelybetween about 2 wt % and about 20 wt % silicon; alternatively betweenabout 2 wt % and about 15 wt %; alternatively between about 2 wt % andabout 10 wt %; alternatively between about 2 wt % and about 8 wt %;alternatively between about 2 wt % and about 6 wt %; alternativelybetween about 2 wt % and about 5 wt %; alternatively between about 3 wt% and about 20 wt % silicon; alternatively between about 3 wt % andabout 15 wt %; alternatively between about 3 wt % and about 10 wt %;alternatively between about 3 wt % and about 8 wt %; alternativelybetween about 3 wt % and about 6 wt %; alternatively between about 3 wt% and about 5 wt %; alternatively between about 4 wt % and about 20 wt %silicon; alternatively between about 4 wt % and about 15 wt %;alternatively between about 4 wt % and about 10 wt %; alternativelybetween about 4 wt % and about 8 wt %; alternatively between about 4 wt% and about 6 wt %.

In various embodiments, a precursor fiber may comprise a layer ofmodifying agent reaction product on the outer skin of the fiber and anasymmetric modified CMS hollow fiber membrane may comprise a layer ofresidual modifying agent reaction product. For example, a precursorfiber that is treated with modifying agent may comprise a layer ofsilicon-containing material on the outer skin of the fiber. Similarly,the asymmetric modified CMS hollow fiber membrane may comprise a layerof a residual silicon-containing material on the outer skin of thefiber. Alternatively, where a metal-containing modifying agent is usedin the pre-pyrolysis treatment of the precursor fiber, the treatedprecursor fiber may comprise a layer of metal-containing material on theouter skin of the fiber and the asymmetric modified CMS hollow fibermembrane may comprise a layer of residual metal-containing material onthe outer skin of the fiber.

The layer of modifying agent reaction product provides the treatedprecursor fibers with a non-stick property that is desirable for thepyrolysis of fibers in bunches. Thus, embodiments of the presentinvention are directed to a precursor polymer fiber comprising amechanical barrier layer that prevents the precursor fiber from stickingto other precursor fibers when the fibers are heated to a temperatureabove the glass transition temperature of the polymer material.

Embodiments of the present invention are also directed to bundles ofasymmetric modified CMS hollow fibers, wherein the modified CMS hollowfibers are substantially free from sticking to one another afterpyrolysis. The modified CMS hollow fibers preferably comprise a layer ofresidual modifying agent reaction product, such as a silica material, onthe outer skin surface. As described earlier, however, it is preferredthat the layer of residual modifying agent reaction product on amodified CMS hollow fiber membrane be thin in order to minimizeinterference with gas flow through the outer skin of the asymmetricmodified CMS hollow fiber membrane.

Examples 19 to 70

The procedure of Example 7 was carried out using a number of differentprecursor polymers and modifying agents. As noted, additional polymerscontemplated for use include:

-   P1. 6FDA:BPDA-DAM-   P2. 6FDA:BTDA-DAM-   P3. 6FDA:DSDA-DAM-   P4. 6FDA:ODPA-DAM    As noted, suitable modifying agents may include-   M1. vinyl triethoxy silane-   M2. vinyl tripropoxy silane-   M3. vinyl tributoxy silane-   M4. divinyl dimethoxysilane-   M5. divinyl diethoxysilane-   M6. tetra methoxy titanium-   M7. titanium methoxypropoxide-   M8. tetrapropoxy titanium-   M9. tetraethoxy titanium-   M10. tetra methoxy vanadium-   M11. vanadium methoxypropoxide-   M12. tetrapropoxy vanadium-   M13. tetraethoxy vanadium

Example Precursor Modifying No. Polymer agent 19 P1 M1 20 P1 M2 21 P1 M322 P1 M4 23 P1 M5 24 P1 M6 25 P1 M7 26 P1 M8 27 P1 M9 28 P1 M10 29 P1M11 30 P1 M12 31 P1 M13 32 P2 M1 33 P2 M2 34 P2 M3 35 P2 M4 36 P2 M5 37P2 M6 38 P2 M7 39 P2 M8 40 P2 M9 41 P2 M10 42 P2 M11 43 P2 M12 44 P2 M1345 P3 M1 46 P3 M2 47 P3 M3 48 P3 M4 49 P3 M5 50 P3 M6 51 P3 M7 52 P3 M853 P3 M9 54 P3 M10 55 P3 M11 56 P3 M12 57 P3 M13 58 P4 M1 59 P4 M2 60 P4M3 61 P4 M4 62 P4 M5 63 P4 M6 64 P4 M7 65 P4 M8 66 P4 M9 67 P4 M10 68 P4M11 69 P4 M12 70 P4 M13Treatment of Thermally Re-Arranged Polymer Membranes

The pores and channels within a polymer film or fiber typically have awide range of sizes, which render the polymer structures generallyunsuitable for gas separation applications. In various embodiments,pyrolysis of a polymer material forms a carbon molecular sieve materialhaving ordered pores. However, certain polymers may be treated to renderthe polymer itself suitable for gas separation applications. Thermallyre-arranged polymer membranes, also known as TR polymer membranes or TRpolymer fibers, remedy the problem of variable pore sizes by thermallydriving spatial rearrangement of rigid polymer chain segments in theglassy phase in order to produce pores having a more controlled size.These changes in the polymer structure are said to increase permeabilityand selectivity properties, rendering the polymer suitable for gasseparation.

Preferred thermally re-arranged polymer membranes comprise aromaticpolymers that are interconnected with heterocyclic rings. Examplesinclude polybenzoxazoles, polybenzothiazoles, and polybenzimidazoles.Preferred thermally re-arranged polymer precursors comprise polyimideswith ortho-positioned functional groups, such as for example HAB-6FDA, apolyimide having the following structure.

The phenylene-heterocyclic ring units in such materials have rigid chainelements and a high-torsional energy barrier to rotation between the tworings, which prevents indiscriminant rotation. Thermal re-arrangement ofthese polymers can thus be controlled to create pores having a narrowsize distribution, rendering them useful for gas separationapplications.

The temperature at which the thermal rearrangement occurs is generallylower than the temperatures used for pyrolysis, as pyrolysis wouldconvert the polymer fiber into a carbon fiber. Polyimides, for example,are typically heated to a temperature between about 250° C. and about500° C., more preferably between about 300° C. and about 450°. Theheating of the polymers generally takes place in an inert atmosphere fora period of a number of hours. Although the polymer is not subjected tothe same stresses of pyrolysis, heating of the polymer at a temperaturesufficient to cause thermal re-arrangement also results in undesirablepore collapse.

Accordingly, embodiments of the present invention are directed to thetreatment of a polymer material with a modifying agent prior tothermal-rearrangement, wherein the treatment is effective to restrictthe undesirable pore collapse of the thermally re-arranged polymermaterial. Treatment of the polymer material is performed in the samemanner described above with respect to treatment of polymer precursorfibers that are then pyrolyzed to form asymmetric CMS hollow fibermembranes. The difference being, of course, that the treated polymermaterial is subjected to thermal re-arrangement, as is known in the art,as opposed to pyrolysis. Embodiments of the present invention are alsodirected to a thermally re-arranged polymer material having a restrictedpore collapse, such as one that is treated with a modifying agent asdescribed herein. It can be seen that the described embodiments provideunique and novel treatment processes, asymmetric modified CMS hollowfiber membranes, and thermally re-arranged polymer membranes that have anumber of advantages over those in the art. While there is shown anddescribed herein certain specific structures embodying the invention, itwill be manifest to those skilled in the art that various modificationsand rearrangements of the parts may be made without departing from thespirit and scope of the underlying inventive concept and that the sameis not limited to the particular forms herein shown and described exceptinsofar as indicated by the scope of the appended claims.

What is claimed:
 1. An asymmetric CMS hollow fiber membrane having asilicon-containing compound within at least one of its substructurepores, the silicon-containing compound stabilizing the poroussubstructure morphology of the asymmetric CMS hollow fiber membrane byrestricting pore collapse during pyrolysis.
 2. The asymmetric CMS hollowfiber membrane of claim 1, wherein the silicon-containing compound isporous.
 3. The asymmetric CMS hollow fiber membrane of claim 1, whereinthe asymmetric CMS hollow fiber membrane contains between about 0.1 mol% and about 10 mol % of silicon, wherein silicon is not present in apolymeric precursor fiber from which the CMS hollow fiber membrane isprepared.
 4. The asymmetric CMS hollow fiber membrane of claim 3,wherein the asymmetric CMS hollow fiber membrane contains between about0.1 mol % and about 3 mol % of silicon.
 5. The asymmetric CMS hollowfiber membrane of claim 1, wherein the asymmetric CMS hollow fibermembrane contains between about 0.1 wt % and about 20 wt % silicon,wherein silicon is not present in a polymeric precursor fiber from whichthe CMS hollow fiber membrane is prepared.
 6. The asymmetric CMS hollowfiber membrane of claim 5, wherein the asymmetric CMS hollow fibermembrane contains between about 1 wt % and about 10 wt % silicon.
 7. Theasymmetric CMS hollow fiber membrane of claim 1, comprising a CO₂permeance of at least 50 GPU and a CO₂/CH₄ selectivity of at least 60when subjected to a mixed feed containing 50 mol % CO₂ and 50 mol % CH₄at 150 psi and 35° C.
 8. The asymmetric CMS hollow fiber membrane ofclaim 7, further comprising a CO₂ permeance of at least 60 GPU whensubjected to a mixed feed containing 50 mol % CO₂ and 50 mol % CH₄ at150 psi and 35° C.
 9. The asymmetric CMS hollow fiber membrane of claim7, further comprising a CO₂/CH₄ selectivity of at least 70 whensubjected to a mixed feed containing 50 mol % CO₂ and 50 mol % CH₄ at150 psi and 35° C.
 10. A process for separating at least a first gascomponent and a second gas component, comprising: (a) providing theasymmetric carbon molecular sieve hollow fiber membrane of claim 1, and(b) flowing a mixture of at least a first gas component and a second gascomponent through said membrane to produce (i) a retentate stream havinga reduced concentration of a first gas component, and (ii) a permeatestream having an increased concentration of a first gas component. 11.The process of claim 10, wherein the first gas component is CO₂, H₂S, ora mixture thereof and the second gas component is CH₄.
 12. The processof claim 10, wherein the first gas component is ethylene, propylene, ora combination thereof and the second gas component is ethane, propane,or a combination thereof.
 13. The process of claim 10, wherein the firstgas component is oxygen and the second gas component is nitrogen. 14.The process of claim 10, wherein the first gas component is carbondioxide and the second gas component is nitrogen.
 15. The process ofclaim 10, wherein the first gas component is nitrogen and the second gascomponent is methane.
 16. A process for separating acid gas componentsfrom a natural gas stream comprising (a) providing the asymmetric carbonmolecular sieve hollow fiber membrane of claim 1, and (b) contacting anatural gas stream with said membrane to produce (i) a retentate streamhaving a reduced concentration of acid gas components, and (ii) apermeate stream having an increased concentration of acid gascomponents.
 17. The process of claim 16, wherein the acid gas componentscomprise CO₂, H₂S, or mixtures thereof.
 18. A carbon molecular sievemodule comprising a sealable enclosure, said enclosure having: aplurality of carbon membranes contained therein, at least one of saidcarbon membranes being the asymmetric carbon molecular sieve hollowfiber membrane of claim 1; an inlet for introducing a feed streamcomprising at least a first gas component and a second gas component; afirst outlet for permitting egress of a permeate gas stream; and, asecond outlet for permitting egress of a retentate gas stream.